(Crop) residue burning
Many farmers burn rice, maize, other crop residues, and natural vegetation in the field. While this is often done to help control diseases or insects, or to simplify the next season’s fieldwork, it destroys the litter layer and results in a diminished amount of organic matter going back into the soil. Additionally, the fire’s heat decreases moisture in the soil.
Burning also wipes out organisms that live in the litter layer and surface soil. In order for decomposition to happen in the future, energy has to be expended to rebuild the microbial community before nutrients can be released from plants.
Some farmers employ burning to better the quality of lands for grazing, believing that this will eliminate the dry, inedible parts and encourage fresh grass to sprout.
It used to be believed that when surface crusting resulted in low infiltration rates for soil, mechanical weeding with tined or disc implements and shallow soil tillage would break up the crust and promote infiltration by rainfall. This has a number of disadvantages: this practice has to be repeated after each rainstorm for soils that are susceptible to crusting, organic matter can be lost, and the soil can be compacted. In other words, using tillage to increase infiltration rates can result in degradation of the soil.
On the other hand, using tined or disc implements for shallow tillage on a regular basis, in order to increase surface porosity and rainfall infiltration through breaking up surface crusts is not a recommended practice. Any increases in the porosity of the surface are temporary, and for soils that are susceptible to crusting the tillage will have to be conducted after each rainstorm. Pore spaces in the soil are disrupted by tillage, and particularly the use of discs frequently results in compaction, which can hamper root growth and the percolation of rainwater. The loss of organic matter in the soil is also sped up by tillage, which can lead to soil architecture deterioration and a decrease in the number and stability of pores in the soil that permit root growth and rainwater movement. Therefore, regular tillage isn’t generally recommended as an answer to the question of restricted infiltration resulting from low porosity of the soil surface.
Increased tillage usually leads to soil pulverization, and the small particles produced can be easily washed away by rain showers’ runoff. When these very fine organic or clay particles clog micropores at the surface of the soil, they form a film-like, thin layer (i.e. surface sealing). Because of this impermeable, continual layer at the surface, rainwater can’t infiltrate and runoff happens more easily.
As soil loses the cover provided by crop residues, mulches, and vegetation, it is more exposed to the impact of rain. When raindrops hit bare soil, the impact’s kinetic energy separates individual soil particles from clods; such particles can clog pores at the surface and form many impermeable, thin sediment layers at the surface, known as surface crusts. These layers can be anywhere from a few millimeters to more than a centimeter thick, and generally consist of silty or sandy particles. Such crusts hamper the infiltration of rainwater. The breakdown of soil aggregates into smaller particles is dependent on the aggregates’ stability, which in itself relies heavily on the content of the organic matter.
Soil pores can also be reduced or even destroyed by the trampling of animals and the use of implements and machinery. Soil that is compacted (soil compaction module) provides inadequate space for the movement and storage of soil air and water. Root growth and soil animals are restricted as well. The most significant impact is that large continuous soil pores are reduced or eliminated, which leads to poor infiltration of water, slower drainage, and reduced aeration that promotes healthy growth of roots and uptake of nutrients that lead to better crop yield.
Significantly, the compaction of soil is frequently not detectable at the soil surface and thus is not considered to be a problem. For those farms where soil compaction doesn’t pose a serious issue, crops with taproots (e.g. pigeon pea, radish, or sunflower) can be introduced into the rotation to help break up the hard layer.
As water drains beyond the rooting zone of the crop, it can reach the groundwater and assist in maintaining the water level of streams, rivers, and wells. Unfortunately, the water that is “lost” by such drainage could have been utilized in the production of crops. When rainfall surpasses the amount of water needed to bring a rooting zone to field capacity, deep drainage occurs. Occasionally, deeper cracks at the surface of clay soils can cause deep drainage during the dry season. For soils that have montmorillonitic clay, there are often cracks with depths of 30-60 cm or more. If it rains while these cracks are present, some water will rapidly move to the bottom of the cracks. On the other hand, if a crack is not open to the surface, water will not enter no matter how much it rains. For instance, if soil cracked to 30 cm is tilled to a 10-cm depth, any precipitation that occurs will predominantly be retained in the upper third unless the rainfall is significant; even in that case, only capillary and gravitational forces will move the water downward. However, if the land is not prepared, some water will move rapidly to the cracks’ bottom and be less available for evaporation.
Additionally, drainage that goes beyond the rooting zone can be encouraged by biopores: continuous pores of widths of 0.5 mm and greater that are formed by ants, earthworms, and termites, and that go from the soil surface to the subsoil. The amount of water lost through deep drainage is less in fine-textured soils, as compared with coarse-textured soils.
Overgrazing, monocropping (the cultivation of a single crop each year), and deforestation are all practices in management that can result in a significant reduction of soil’s moisture content. Such practices lead to reduced porosity of soil, which then lessens the ability of rainwater to infiltrate the soil and the ability of the soil to retain any moisture that it receives.